Setting namespace multiple times is not supported.

[sn01615]

...
$router->setNamespace('Ncontrollers');
$router->get('/aaa/bbb', 'AController@ccc');
...
$router->setNamespace('Ncontrollers\\Api');
$router->get('/aaa/ddd', 'BController@eee');
...

[hozzaq]

you can do this

$router->get('/aaa/bbb', '\Ncontrollers\AController@ccc');
$router->get('/aaa/ddd', '\Ncontrollers\Api\BController@eee');

[bramus]

What @hozzaq said. Evaluation of the defined namespace only happens upon execution of the set callable on the route.